1. Field of the Invention
This invention relates to an apparatus for the production of a double-walled synthetic plastic tube characterized by a smooth inner tube which abuts the nadir of the corrugations of the outer tube within the outer tube. This invention is particularly concerned with an improvement which insures that the inner tube is smooth and has a constant thickness about its circumference. This invention is particularly directed to an apparatus which insures that at any point of the tube the thickness of the tube is not greater on one side of the inner tube than it is on the opposite side thereof or at another point along its circumference. This invention is especially concerned with the production of a double-walled corrugated plastic tube having an inner tube which abuts the corrugations of the outer corrugated tube on the inside thereof which inner tube is of an especially small thickness.
2. Discussion of the Prior Art
It has already been proposed to produce double-walled synthetic plastic tubes having an annularly or helically corrugated outer wall and a longitudinally smooth inner wall by extruding two concentric tubes in one operation, the other tube being extruded from a first annular extrusion die into a mold cavity formed by divided molds having transverse internal grooves and forming two cooperating endless recirculating trains of mold halves which close to complete the mold cavity, said tube being made to conform with the shape of the transverse grooves, whereas the inner tube is extruded from a second die extending into the interior of the mold, and is pushed into contact with the fully formed outer tube.
It is also known in apparatus for the production of such double-walled tubes to attach to the mandrel of the die for extruding the inner tube a mandrel extension having a diameter at least approximately corresponding to the internal diameter of the outer corrugated tube so that this extension will operate to press the inner tube against the outer tube so that the two tubes are satisfactorily bonded together by fusion (German Patent Specification No. 1,704,718 published prior to acceptance).
In these known arrangements difficulties arise in connection with a sufficiently accurate central location of the mandrel extension inside the mold to ensure that the wall thickness of the resultant double-walled tube is uniform around its entire circumference. Moreover, irregularities in the extrusion of the plastics stream may cause the mandrel extension to be forced aside and to remain in this position with the resultant production of a tube in which the wall thickness is not uniform all around.